1. Field of the Invention
This invention relates generally to surge suppressor circuits to provide protection to sensitive electronic or electrical equipment from high energy transient voltage events, and more specifically to a circuit that employs more than one suppressor element with one such element initially actively connected in the circuit, whereby the device automatically switches to a subsequent suppressor when the original suppressor element is close to failing or has failed. At least one suppressor element is not exposed to transient voltage events at any one time.
2. Discussion of Prior Art
Surge suppressors are devices that protect sensitive electronic and electrical equipment from high energy voltage transients. A number of devices are very susceptible to high voltage, high energy events and require surge suppressors to protect them from the harm the event can cause. There are several devices that fit in the category of xe2x80x9csurge suppressor.xe2x80x9d Inductor/capacitor networks and avalanche diodes are a couple of examples. However, only a few of the available devices have the speed, size, cost effectiveness, and energy handling capability to be an effective device for protecting a system from these events. Metal oxide varistors (MOVs) and silicon avalanche diodes (SADs) are two products that are frequently used because they currently have high performance in all of the above categories. However, MOVs tend to wear down with each transient voltage event until they eventually fail, and SADs offer lower energy handling capabilities at comparable cost. When either of these devices fails the result can be an explosion that can sometimes lead to fire. Care must be taken by users to control the results of the eventual failure of such devices.
The most common method of controlling the results of a suppressor failure is to place a fuse in series with the suppressor, where the fuse opens when high current goes into the suppressor for long periods of time (5-10 ms, for example). Various methods of detecting that the fuse is blown are also commonly used to provide a warning signal to the user that the load is no longer being protected. A disadvantage of the fuse is that only high current is detected and interrupted.
Another conventional method incorporates a low melting point solder that eventually melts due to the heat generated by a failing suppressor. When the solder melts, the circuit is interrupted in various ways, disconnecting the failed suppressor. There are several disadvantages to this method. Under some circumstances the solder will not melt quickly enough and significant heat, smoke and explosion can be produced during that delay. Further, this method does not react to high currents directly and quickly. This results in the requirement of an additional fuse for this purpose, which adds expense and consumes space.
There are a few other methods that also operate by detecting a failed suppressor as the warning that the load is no longer protected. Many of these methods are somewhat sophisticated and use lasers and smoke detectors. Most of these methods share the concept of the fuse and melting solder techniques in that the suppression device is disconnected and the user of the suppression device is warned that load is no longer being protected.
These prior methods of detecting load susceptibility to high energy voltage transients have a major drawback. The load has no protection from high voltage transients after failure and before servicing. Many users of surge suppression devices do not check their surge suppressors daily or even weekly for warnings that the load is not being protected from transient voltages. During this period, their system is unprotected and the probability of failure of the load has increased dramatically. This indicates a need for early detection of suppressor failure so that the load is never vulnerable to high transient voltages.
One method of detecting when the load is close to being unprotected from high voltage transients is to have several suppressors connected in parallel with one another, each parallel leg being in series with a fusing element connected to the load that is to be protected. This method subjects every suppressor in the device to every high voltage transient that is on the line. The device detects when each suppressor fails and the device indicates that the load is getting close to being unprotected by the reduced number of suppressors still functioning properly. This method is effective at giving early indication of susceptibility of the load to high voltage transients. However, there are drawbacks to this method. Because the suppressors are in parallel and all are subjected to high voltage transients, all of the components are degraded together. This means that the amount of remaining protection available is variable and it is difficult to predict when the load will be unprotected.
Another approach sometimes used is a surge suppressor and capacitor filter combination. This method affords a high degree of protection for the load and increases the life span of both components. The suppressor elements in this type of system will eventually fail, just as in previous methods. However, when the suppressor has failed the capacitor filter is still in place to protect the load, although to a lower degree, until the suppressor is replaced. The drawback of this method is that the filter network may not have the capability of shunting enough energy to protect the load to the extent that is necessary.
A major purpose of the invention is to provide a surge suppression device that is relatively fast, small, cost effective, and has a high energy handling capability. The suppressor circuit also controls the results of failure and continues to protect the load from high voltage transients reliably, while at the same time warning the user that the suppressor protection has been degraded to a pre-defined amount. A function is to provide continual protection to the load from high voltage transients while giving the user adequate time to replace the suppressor element or circuit.
The invention accomplishes these purposes in a single package that incorporates two or more suppressor elements dedicated to protecting a sensitive electrical circuit. The suppressor elements are configured in such a manner that at least one supplemental suppressor element is isolated, inactive, and protected from the harmful transients that the primary suppressor element is subjected to. The performance characteristics of the supplemental suppressor are therefore preserved until the backup is needed and the supplemental suppressor device is connected into active use in the circuit.
In addition, the suppressor circuit is configured to control the results of a failed primary surge suppressor element. This is accomplished using a combination of methods of detecting failure or imminent failure of the primary suppressor element. In one contemplated arrangement this combination consists of a thin conductive material under tension or torsion, that material being soldered to the suppressor element termination using a relatively low melting point temperature solder. This combination accomplishes protection of the suppressor circuit by detecting both high temperature and high current conditions corresponding to failure or imminent failure of a suppressor element.
The preferred method of embodying this invention uses a low melting point, electrically conductive solder to attach a conductive piece which forms a conductive path between a switching mechanism and a suppressor element lead. The conductive piece is sized and configured to respond to high current conditions by fusing. This connecting piece either detaches from the suppressor lead due to the lead temperature of the suppressor rising above a pre-defined point, thereby melting the solder, or due to fusing of the conductive piece due to high current flow through the conductive piece. In either case, this allows the switching mechanism to actuate, thus disconnecting the first suppressor element from the circuit being protected and connecting a subsequent suppressor element to the circuit being protected. The solder has a melting point that is closely correlated with the amount of heat produced and lead temperature presented by a failing suppressor element.
This same method of disconnecting a suppressor element and connecting a fresh suppressor element in the circuit can be cascaded any useful number of times to continually refresh suppressor protection for the load.
A failing suppressor element device can produce expanding gas and propel particles which should be exhausted or controlled. In one embodiment of the invention, the gas and particles are channeled in a non-direct route through the enclosure to an exhaust port. Most particles are kept within the case and only the expanded gas and a small amount of soot exits the case. This avoids damage to adjacent assemblies and reduces the risk of fire.
In one embodiment, the fusing action of the conductive piece is assisted by channeling the expanding gas and propelled particles from a failing suppressor in a mainly perpendicular direction through the conductive piece, thus aiding in quickly clearing the circuit.
It is important that the user be able to observe the functional status of a suppressor device. In combination with the apparatus a sensing means and indication means can be included. The sensing means can be any common sensor device that can detect movement such as a microswitch or a photo emitter/transistor. This sensor can be coupled to any number of display types such as indicator lights, a logic circuit driven display or a field relay intended to convey the status to other equipment.
In another embodiment, the indicators are connected to the switch devices and are visible or not-visible to the user, depending on the position of the switches, thus mechanically indicating status.